The steps outlined here make many assumptions about both your operating environment and your understanding of the Linux OS and services running on Linux. I am using the current LTS Ubuntu distribution 16.04 running in the cloud on a DigitalOcean Droplet. These steps should work just fine if followed closely and in order... but you know sh!& never works out the way you plan in Linux. I offer some troubleshooting advice, Google and Stack are your friends ask them for help.
ssh-add <path-to-ssh-key>
and enter key passphrase when prompted. // ex path... /users/you/.ssh/id_rsa_digital-oceanssh root@<server-url-or-IP>
// should login without prompting for passphrase
adduser
and follow prompts